1. Field of the Invention
The present invention relates to a contact charger for charging an image carrying body while being brought into contact with the image carrying body, and an image forming apparatus comprising the contact charger.
2. Description of the Related Art
A brush charger disclosed in U.S. Pat. No. 5,012,282 is known as a conventional contact charger. The brush charger comprises a conductive base and conductive brush fibers in contact with the conductive base. A charging voltage from a power supply is applied to the surface of a photoconductive drum.
A brush charger of this type has a drawback of unevenness in charge, as will be described below.
First, if the density of brush fibers implanted into the conductive base is low, the probability of the brush fibers being in contact with the photosensitive drum is also low, thereby causing minute unevenness in charge. Since it is difficult to manufacture a brush charger having an increased density of the brush fibers, the width of the brush is increased to increase the probability of the brush fibers being in contact with the photosensitive drum. However, it is difficult to increase the probability of the brush fibers being in contact with the photosensitive drum per unit time, in consideration of the space in which the charger is mounted.
Secondly, the brush charger is likely to catch fine toner after cleaning or dust adhered to the surface of the drum. Further, in a static boundary of the drum (the portions of the drum which correspond to the front and the rear ends of a paper sheet), the toner or dust adhered to the brush is discharged to the surface of the drum again. The toner or dust adheres to an output image as contaminant.
Thirdly, brush fibers may be removed from the base owing to incomplete implanting of the fibers in the manufacturing step, or mechanical or static factors during an operation. Removed fibers may be transferred to apparatuses in the other image-forming processes, e.g., a developing apparatus or a transferring apparatus, resulting in various problems such as a leak or a minute gap.
Fourthly, brush fibers are generally made of rigid chemical fibers. Since rigid fibers abut on the photosensitive drum, the surface of the photosensitive drum is etched by the sharp edges of the fibers. The thickness of the etched portion of the drum is reduced, resulting in various problems, e.g., reduction in the charge potential and imperfect cleaning due to local etching. Thus, the lifetime of the drum may be greatly reduced. Further, the etched-off portion of the drum surface may adhere to the distal ends of the fibers, with the result that uniform charge cannot be achieved.
Finally, in a brush charger, the amount of deformation of the end portions of the fibers and the charge potential have a proportional relationship. Therefore, to obtain a constant potential, the amount of the deformation must be uniform over the length of the drum. However, it is difficult to cut fibers precisely to the extent that the charge potential is not influenced. Further, the attachment accuracy of the brush charger with respect to the photosensitive drum and the bent of the fibers may cause charge unevenness, which influences an output image.